In modern computer control systems, it is frequently necessary to reset convert a digital signal (which is used internally in the computer) to a variety of analog signals which are used to directly control or measure the environment. Two conversion devices which are ofter used in manufacturing systems are digital-to-analog converters (DACs) and analog-to-digital converters (ADCs). These units convert between analog signals generated by the environment and the digital signals used by the computer.
Another, perhaps less widely used, conversion device is a digital-to-time converter. This unit accepts a digital signal and produces a proportional time delay. The delay usually appears as a time difference between two pulses appearing at the output of the device or between a trigger pulse and a pulse appearing at the output of the device. Such programmable time delay circuits are often used in automated test equipment and are used to delay digital signals.
Digital-to-time converters have conventionally been fabricated from discrete semiconductor devices. In such devices, the conversion is often performed by comparing a linearly-increasing voltage or current ramp signal to a threshold voltage or current. In one conventional form of a digital-to-time converter, a fixed threshold voltage is set by a precision voltage reference source and the time delay is generated by comparing the threshold voltage to a ramp with a variable slope. The slope of the ramp is set by the value of the digital word to program the device. In another conventional form of the converter, a ramp with a fixed slope is generated and the time delay is obtained by comparing the ramp voltage to a variable threshold whose level is set in accordance with input digital word.
In either of the above variations, when the value of the ramp voltage equals the value of the threshold voltage a pulse signal is generated. If a pulse signal is generated at the start the ramp signal, the time elapsing between the two pulse signals represents a delay which depends on the value of the digital input word. The starting pulse may also be the trigger pulse which is used to start the ramp signal generation.
In a conventional digital-to-time converter designed with discrete devices, the internal ramp signal is created by charging a capacitor with a stable current generated by placing a precision voltage reference source across a precision resistor. Once a stable charging current has been established, the voltage across the capacitor provides a stable ramp output.
Such a ramp generator is usually reset by means of a shorting transistor connected in shunt across the capacitor. When the shorting transistor is turned "on", the voltage across the capacitor is returned to zero, resetting the circuit. The shorting capacitor is normally controlled by the output of a flip-flop or other memory circuit which determines whether the circuit is operational or rest in response to the application of set or reset signals.
The problem with the conventional arrangement is that the reset signal which operates the flip-flop must propagate through the flip-flop to turn on the shorting transistor and reset the circuit. Since the flip-flop contains many transistors and other elements, the time consumed between the receipt of a reset signal at the flip-flop input and the actuation of the shorting transistor is usually significant and thus the reset time of the entire circuit is increased by the propagation delay of the flip-flop. Since the reset time of the circuit is a substantial portion of the operating cycle of the circuit, the entire operational frequency is reduced.
Accordingly, it is an object of the present invention to provide a reset circuit for a ramp generator which can operate at high speed.
It is another object of the present invention to provide a reset circuit for a ramp generator in which the reset function operates at a higher speed than conventional circuits.
It is yet a further object of the present invention to provide a reset circuit for a ramp generator which can be easily fabricated in a monolithic integrated circuits.
It is yet a further object of the present invention to provide a reset circuit for a ramp generator which can be easily intergated with existing control circuitry.